The present disclosure relates to optical tweezers, in particular an apparatus and method for controlling a plurality of optical traps.
Optical tweezers may be considered one of the pillars of the single-molecule toolbox. The method typically comprises application of one or more highly focused laser beams (“trapping beams”) in a sample volume to form optical traps. The physical principle can be explained by the momentum transfer (force) between the incident photons and the trapped objects that occurs upon refraction. For example, when the geometry of the experiment includes a spherical object (“microsphere” or “bead”) having a refraction index higher than the surrounding medium (glass and water for example) the generated optical forces creates a stable three dimensional optical trap. Optical tweezers can be used for example in remote manipulating of micrometer-sized objects. One application comprises attaching a molecular strand such as a DNA molecule between two optically trapped microspheres. In this way it is possible to manipulate and control the conformation of the DNA as well as apply and measure forces on it. This information can be used to study the DNA's mechanical properties and its interaction with proteins.
Traditionally, one or two optical traps are used at a time. However, in many biological circumstances, proteins interact with multiple sections of DNA at the same time (e.g. proteins that repair DNA breaks or are involved in DNA compaction by forming loops in DNA). Such interactions cannot be studied accurately and in a controllable way with only two optical traps. The canonical optical tweezers implementation does not allow to position different DNA molecules in contact in a controllable way while simultaneously measuring the tension on each of them. Furthermore, the data throughput of such implementations is low, since only one DNA molecule is studied at a time.
There are reports of instruments that are able to manipulate different DNA molecules at the same time. See for example, M. C. Noom, B. van den Broek, J. van Mameren, G. J. L. Wuite; “Visualizing single DNA-bound proteins using DNA as a scanning probe”; Nature Methods, 4, 1031-1036 (2007). This prior art involves a design in which the optical tweezers is time-shared between multiple locations (the position of a single laser beam is alternated among separate locations using acoustical-optical deflectors-AOD), thereby producing multiple optical traps. However, in time-shared optical tweezers the trapping in each of the positions is not continuous in time, making them unstable, their force-calibration problematic and in general not suitable for many scientific applications.
As another example, see I. De Vlaminck, M. T. J. van Loenhout, L. Zweifel, J. den Blanken, K. Hooning, S. Hage, J. Kerssemakers, and C. Dekker; “Mechanism of homology recognition in DNA recombination from dual-molecule experiments”; Molecular Cell, 46, 616 (2012). In this prior art dual optical tweezers are combined with Magnetic tweezers. In the configuration one of the DNA molecules is anchored between a magnetic sphere and a glass surface. The second DNA molecule is held in solution between the two optically trapped spheres. However, the use of surface-bound DNA in the Magnetic tweezers limits the flexibility and ease of use of the instrument, making this approach for example less compatible with a microfluidics platform.
Accordingly, there remains a desire to continuously manipulate and monitor a multitude of optical traps in a reliable fashion.